Building a Custom 9-Inch with Currie

We all know somebody with a pretty wild ride. You know the guy who crammed a HEMI into his Ford T-Bucket, or a 454 Chevy big block into an AMC Gremlin. Maybe that guy is even you. While a lot of time and effort is spent on the engines and transmissions of such projects, the unsung hero is the rear end setup. All that power will never make it to the pavement if the rear differential can’t handle it. But how do you find a rear end tough enough to handle all the horsepower and torque of a custom, hardcore-offbeat-project like our ’82 Caprice Classic Wagon?

This super sleeper project has already been modified well beyond its stock specifications, including a full Spohn Suspension to improve handling as well as a fully-built engine to replace the ancient 305 small block, which, on the dyno, the new engine made an impressive 650 horsepower and 613 ft-lbs of torque. Because of this, we knew the stock GM 10-bolt rear end would never be able to handle that kind of power.

We went to the rear end experts, Currie Enterprises, a company that has plenty of experience building custom rear ends. Currie has decades of experience in making housings for all kinds of custom applications. From street and strip automotive racing to rock crawling, off-roading, mining and military applications, Currie can do it all. That sounds like an awful lot of bases to cover, but Brian Shephard at Currie explained that automotive rear end housings account for only about 40% of Currie’s business.

Dropping the original 10-bolt was first on our list. Since we were kind of reinventing the wheel here, we had to get Currie all the original measurements off the original bracketry and according measurements.

“We do a lot of industrial stuff for a lot of odd applications,” Shepard explained, including rear ends for large lawn mowers (such as the kind you might see mowing the grass along a highway). They also make housings for a few Disneyland rides. Figuring that they’d be able to help out with a custom rear end for our wagon, they needed some critical information before they could start fabricating anything.

Custom Rear Ends For All

“The first thing we need to know is what kind of car you need a rear end for,” says Shepard. “We see a lot of odd things come through the shop; ’49 Oldsmobiles, mid-60’s Pontiacs, and a lot of old Fords.” Currie is a large operation, and they carry a lot of rear ends in stock. They base many of their rear ends on refurbished Ford 9-inches, 8.8-inches, and GM 12-bolt rears. But some things require something far more custom than what they have in stock. After all, how many people out there are turning Caprice wagons into tire-shredding street racers? The answer is not many.

Since Currie needed the original brackets from the rear, the housing to sent to Currie’s facility to measure the proper placing and cut off the original brackets. Using the original hardware, Currie cleaned up the old brackets to new.

Imperative to our wagon’s new 9-inch was getting all the factory brackets aligned appropriately with our stock perch locations. Currie’s team meticulously measured and tweaked accordingly.

“Once we know what kind of car you’ve got, we need to know what you intend to do with it, and how much horsepower you are planning to have,” Shepard continued. “We don’t want to overbuild or under-build a rear end for you. We want to get it right for you the first time.” Since our wagon is pushing out well over 600 horsepower and 600 ft-lbs of torque, we needed something beefy, but not over-the-top. It would definitely require 35-spline axles and as we intended to run 4:10 gears, that would also factor in to the overall finished product.

When you are dealing with a heavy-duty rear end, sometimes you need to apply some brute force.

Getting The Measurements

If you’re planning on replacing the rear end in an otherwise already complete car, or you have a fairly common car (like a Mustang or Camaro) you can tell Currie a few key details that can help them build the right rear end for you. “There are a few questions we will ask right off the bat, like car, application, and desired horsepower,” says Shepard. “After we get that information, there are a few other things we need to know.”

For example, tire diameter plays an important role when determining the overall length of the rear end. Is the car going to be tubbed, mini-tubbed, or filling up the entire width of the stock fenderwells? Since our wagon was getting 275/60/15 tires with 15-inch wheels, we knew we wanted a slightly narrower rear end than stock. In addition to knowing tire diameter size, the RPM range of the camshaft can play a critical role in determining what kind of rear end you need.

It is important to know if your pinion is off center or not, especially when it comes to putting a Ford rear end in a Chevy vehicle.

Our engine is running a COMP Cams Xtreme Energy hydraulic roller cam, which has an optimal operating range between 2800 and 6100rpm. Another critical question is what kind of transmission you intend to use; automatic or manual? Maybe it is an automatic with a manual valve body, or perhaps something even more awkward, like the classic “three on the tree,” a manual transmission with the shifter on the steering column. Our wagon is getting an automatic TCI 4L80E transmission, so we passed all this information along to Currie.

Overall width (wheel-to-wheel)

Pinion location

Wheel bolt pattern

There are three dimensions Currie needs to know above all else; overall width, pinion location, and wheel lug bolt pattern. We’ll take each of these in turn, starting with overall width. Rather self-explanatory, the measurement is the width of the rear end housing from the back of one wheel to another. Take into account that both the fenders and the frame rails require a minimum of one inch of clearance to avoid tire or wheel rubbing. This is a fairly easy measurement to get, requiring just a tape measure to accurately gauge.

The second important measurement is the pinion location. What is the pinion? The pinion is the smaller diameter gear in the differential. The tail end of a pinion ends in a knurled shaft that is attached to a universal joint jutting out of the differential housing. Remember, not every pinion is centered on the rear end housing. In fact, the most popular rear end used by Currie, the Ford 9-inch, is off centered, and thereby making the stock pinion location such an imperative measurement. Depending on the application, it can be off-center by a couple of inches or more. So it is important to know how far off center your pinion is, so it can line up with the driveshaft correctly. It also allows for the correct clearances for the brackets on narrowed rear ends.

Knowing your bolt pattern is also important, so Currie can get it right the first time around.

Finally, Currie needs to know the wheel lug bolt pattern. Just about every brand of vehicle has a different wheel lug bolt pattern, and to determine the diameter requires just a little bit of math. First, you need to know the number of lugs, usually either four or five lugs. The easiest way to measure the lug pattern is to measure from one stud to the opposing stud. Currie has a handy page on their website that details these measurements and provides some pictures to help you figure these dimensions out for yourself.

Down And Dirty

For our wagon, we removed the stock 10-bolt rear end and rolled some fat 275/60/25 tires under the fenders to get an idea of where they would sit in the final product. We decided that a stock-width rear end would be fine for our setup, allowing some slightly-wider tires without having to chop and tub our wheelwells. With all of our measurements, we took the old 10-bolt rear end to Currie where they built us a new rear end based on a Ford 9-inch to exactly 65.5-inches, which was the width of our original 10-bolt. Imperative to getting our new Currie mated up to our ’82 wagon were proper brackets. Currie fabricates their own brackets for the near-limitless array of applications. Using our stock 10-bolt as a reference, Currie installed new brackets, as well as the RideTech brackets that would allow us to set up our air suspension once the rear end was in place.

These suspension mounts had to be mounted to fit the rear end to our wagon. To do so, Currie created this custom jig to position them correctly.

Once we got the housing all figured out, we still needed a bulletproof differential. You know, all those fancy gears hidden in the pumpkin that do the actual work. Currie is one of the top distributors for Eaton differentials, the same people who make the popular superchargers found on many aftermarket and factory cars like the Corvette ZR-1 and Shelby GT500. So it is only natural we would go with an Eaton differential in our Currie rear end.

The Difference In Differentials

To get some specifics, we turned to Michael Mulholland who represents Eaton. Very knowledgeable of differentials, Michael explained to us some of the key differences between different kinds of differentials, and how you can choose the right one for you.

The humble exterior of a Detroit Locker. Doesn’t look like much until you get inside.

“Eaton offers a variety of differentials for a variety of different applications,” Mulholland says. Just like rear end housings, your plans for the car can affect the type of differential you put in your vehicle. Eaton actually produces two of the most popular differentials on the market, the Eaton Posi and the Eaton Detroit Locker. “Eaton actually owns the ‘Posi’ trademark,” says Mulholland. “They love the movie ‘My Cousin Vinny’ because Marissa Tomei’s explanation of the Posi system is spot on.” The Posi-traction is a limited-slip differential.

This type of differential transfers power from one wheel to another via carbon disc clutch packs preloaded by a spring assembly. When torque increases, the clamping load on the clutch pack increases and thus transfer the power to the other wheel that is slipping. This allows for tight cornering, and at the same time you can still put down two perfect, smoking strips of burned rubber. “Nobody likes a one-legged burnout,” Mulholland says with a laugh.

An “exploded” view of a Detroit Locker. Notice how both sides are symmetrical, directing equal amounts of power to both wheels.

So what about the Detroit Locker? “This is for more aggressive vehicles, drivers and engines,” says Mulholland. “Professional racers from NASCAR, NHRA, IHRA regularly use Detroit Lockers because they provide 100% torque to both wheels.” Like the Posi, the Detroit Locker can allows for different wheel speeds. “Eaton’s Detroit Locker is a real work horse product, but it is really made for high horsepower and hyper aggressive vehicles.” says Mulholland. “Other people like something a bit more subtle.” The Detroit Locker has proven itself over the years and has acquired a legendary reputation for veteran gearheads who use them regularly, even in street driven cars.

For people who want the biting power of a Detroit Locker, with the limited slip of a Posi, there is the Detroit Truetrac. With no wearable parts, it is essentially a maintenance-free rear end that operates as an open-differentials (i.e. one spinning wheel) until the power is needed. “These differentials last a long time, and often times the axles will break long before the differential.” The Truetrac offers smooth operation and a lot of bite, and it is very popular with the resto-modding crowd.

What a Truetrac looks like on the inside compared to the Locker above. Not so symmetrical, so it can transfer power to the rear wheels when they need it.

As for our wagon? Well in the end, we sent Currie the stock 10-bolt rear end and had Currie cut the old brackets off. They then went ahead and welded new brackets onto a brand new Ford 9-inch rear end for us. Currie is ultra precise in their work, measuring, re-measuring, and re-re-measuring to ensure everything lines up straight. They even put the rear end on a straightener after welding it, as sometimes warping can happen from all that heat. This ensured that our project car would line up just right and keep the wagon straight and true down the drag strip. And to make sure our big block doesn’t shatter any axles, we went with some beefy 35-spline axles over the original 31-splines. This gives us more surface area, spreading out the power and ensuring we don’t bust any parts on our way down the strip.

Currie can custom build rear end housings for just about any application, including our wagon project.

For now, the wagon is getting the tried-and-true Eaton Detroit Locker (Part# DTL-187S160A). After all, he is a bit of an old, clunky car (at least for now) and there is nothing like seeing a station wagon laying down two perfect burnout treads to raise some eyebrows. However, future plans call for the more subtle TruTrac. Eventually. The wagon already has a complete Spohn Suspension, including upper and lower control arms and a steering rebuild kit. The wagon is coming along great, and we can’t wait to get him out on the track!